By Jeremy Lovell

LONDON, Oct 16 (Reuters) - Three British polar explorers will set off from Alaska early next year on a trek to the North Pole to try to establish when Arctic summer sea ice will finally vanish because of global warming.

The sea ice is already receding at a rate of 300,000 square kilometres (115,000 square miles) a year -- equivalent to the area of the British Isles -- but despite some submarine and satellite measurements there is no accurate measure of how rapidly it is also thinning.

"It is thinning very fast. But there is a missing data set for scientists -- how thick is the ice layer," veteran Polar explorer Pen Hadow said on Tuesday. "If we know that, we can predict with much greater accuracy how long it will last."

Estimates of final total disappearance of the summer sea ice range from 16 to 100 years, and the aim of the four-month expedition starting in February is to fine-tune that by getting accurate readings of the ice's thickness from the surface.

And it is not just Polar Bears at risk. As it retreats, so countries surrounding the region are starting to stake their claims on some of the richest untapped mineral and marine resources on the planet.

Russia has already claimed half of the Arctic sea bed where an estimated 25 percent of the earth's known reserves of gas and oil lie, and the summer opening of the Northwest Passage off Canada could cut weeks off east-west sea voyages.

"This is an international scientific endeavour," said Hadow. "Global warming will be about opportunities as well as threats. It is all about how we are going to adapt. What we see in the Arctic Ocean now can only get worse before it gets better."

The three -- Hadow, Ann Daniels and Martin Hartley -- will walk, swim and ski the 2,000 km (1,250 miles) route over some of the toughest terrain in the world in temperatures down to minus 50 Celsius (minus 58 Fahrenheit) towing behind them an ice-penetrating impulse radar.

The specially designed radar will measure and transmit readings of the depth of snow and underlying ice every 20 centimetres (8 inches) -- meaning that during the journey it will have taken 10 million readings.

Although it will take years for scientists to analyse the data, Hadow said the team hoped that when they reach the North Pole in early June they would be able almost immediately to give a preliminary estimate.

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